Only 1 air sensor on HPO rails were functional. When wafer flats were orientated so that the good sensor was missed, the wafer would not stop at the HPO to bake. Both sensors functioning properly as indicated by the LED's on the sensor PC board( underneath the carriage. However, 1 sensor signal was not getting from the sensor PC board to the CPU board. Troubleshot and verified that interconnect was good between the sensor PC board and the CPU board. Installed new sensor PC board and tested sensor feedback to CPU board- All OK ( both sensors recognized by CPU board and sensor states change when flagged) Cycled many wafers and adjusted carriage belt speed. All tested OK.